Basically, one way to test the fuel screw circuit is during a sudden 100% full throttle snap from idle check. The vacuum of the engine will draw fuel INITIALLY from the fuel screw circuit (the progression circuit in delortto speak) and if too lean, the engine will bog and or die. If too rich, the revs will just build slow. Its very easy to detect a lean bog , so thats what you use as a indicator - so you start from a known lean condition and work towards getting rid of it. Ideally, what you want, is enough fuel to prevent a lean condition during this wide open snap of the throttle and "just" eliminate the lean bog and alittle bit more for safety (said in Italian this sounds better)
The safety adder portion should actually be tested as well - you do that with a throttle chop. Find a big steep hill (or a cranberry bog
) that you can climb under WFO throttle and listen for any lean pinging after you chop the throttle at the top. If you hear any pinging, you need to richen the fuel screw more. Use this hill to verify your main jet too while your at it!
My Procedure on PHBL:
Make sure bike is good and warm. Never play with the setting with a cold bike.
Turn fuel screw in to about 1 turn out or so. (or whatever setting you know will produce a bog - you'll know it when you hear it)
Turn down idle speed to very low - enough so it doesnt stall, but very low.
Wack throttle wide open very quickly. Really make sure you get a good fast full turn snap. Listen for obvious lean bog.
richen fuel screw in 1/2 turn increments until the lean bog goes away. It will become a more subtle transition but still there.
repeat with 1/4 turn settings near point where lean bog goes away to improve setpoint accuracy. Once you find that gray zone where the bog is "just" gone - add another 1/4 to 1/2 turn richer to the fuel screw.
I find by lowering the idle way down, this really helps fine tune the "just" gone or subtle transition and I find I can get away with only adding a 1/4 turn*. The PHBL has a very sharp and defined transition so is very easy to set, so even 1/8 turn resolution of the transition is attainable once your used to it. (Unlike a VHST or even a Keihin that are alittle less defined IMO) If the weather gets really hot and humid, your safety adder from setpoint may grow from the 1/4 turn to the 1/2 turn range. (*test on hill - once found use that or be safe and use 1/2 turn)
The above can take some seat of the pants practice, but once you calibrate yourself to your bike, and you become repeatable, its' really a quick and easy thing to set just right. I will test it anytime the weather changes dramatically. Hot and humid vs cold and dry would be at least 1/2 turn different - so worth doing.
Theres a video example of this is one of the jim snell motor video's. probably the last one near the end.
After all is said and done, you should be in the 3 to 3 1/2 turn out range on the fuel screw on a average day near sea level. If not, your either jetted wrong or the carb/jets are dirty. (on a GG 250 -300 anyway)
If done correctly, the bike should have instant throttle response and the revs should drop quick too.
If you goto the NETA Trials school, Stu or I can demonstrate it as well, but im sure you'll pick it right up.